Resonance Raman Spectra of Adsorbed Monolayers Very recently, the exchange reaction between HzO vapor and chemisorbed Cog on ZnO has been r e p ~ r t e d . Those ~~i~~ and present results give clear evidence that the chemisorption energy of HzO is larger than that of COz or "3. Acknowledgment. The authors wish to thank Mr. Munehiko Goto for his assistance in a part of the experiments.
References and Notes (1)J. E. Mapes and R. P. Eischens, J. Phys. Chem., 58, 1059 (1954). (2)L. M. Roev, V. N. Filirnonov, and A. N. Terenin, Opt. Spektrosk., 4, 328 (1958). (3)D. E. Nicholson, Nature (London),186,630 (1960). (4)J. 8. Peri and R. 6. Hannan, J. Phys. Chem., 84, 1526 (1960). (5) N. W. Cant and L. H. Little, Can. J. Chem., 42,802 (1964). (6)J. E. Peri, J. Phys. Chem., 89, 231 (1965). (7)J. J. Fripiat, A. Leonard, and J. 6. Uytterhoeven, J. Phys. Chem., 69, 3274 (1965). (8) M. J. D. Low, N. Rarnasubramanian, and V. V. Subba Rao, J. Phys. Chem., 71, 1726 (1967).
475 (9)M. R. Basila and T. R. Kantner, J. Phys. Chem., 71, 467 (1967). (IO) G. A. Blornfleld and L. H. Little, J. Catal., 21, 149 (1971). (11) G.Blyholder and E. A. Richardson, J. Phys. Chem., 88, 2597 (1962). (12)T. Morimoto, J. Irnai, and M. Nagao, J. Phys. Chem., 78, 704 (1974). (13)J. H. Taylor and C. H. Arnberg, Can. J. Chem., 39, 535 (1961). (14) K. Atherton, G. Newbold. and J. A. Hockey, Discuss. Faraday SOC.,52, 33 (1971). (15)C. C. Chang. L. T. Dlxon, and R. J. Kokes, J. Phys. Chem., 77, 2634 (1973). (16)T. Morimoto, M. Nagao, and F. Tokuda, Bull. Chem. SOC.Jpn., 41, 1533 (1968). (17)T. Morirnoto and M. Nagao, Bull. Chem. SOC.Jpn., 43, 3746 (1970). (18) M. Nagao, J. Phys. Chem., 75,3822 (1971). (19) A. L. Dent and R. J. Kokes, J. Phys. Chem., 73, 3781 (1969). (20) A. A. Tsyganenko and V. N. Filirnonov. Spectrosc. Lett., 5, 477 (1972). (21)T. Morimoto and M. Nagao, J. Phys. Chem., 78, 1 1 16 (1974). (22) K. Nakamoto, "Infrared Spectra of Inorganic and Coordinate Cornpounds", 2nd ed, Wiley-lnterscience, New York, N.Y.. 1970. (23)A. J. Tench and D. Giles, J. Chem. Soc., Faraday Trans. 1, 68, 193 (1972). (24)A. J. Tench, J. Chem. SOC.,Faraday Trans. 1, 68, 197 (1972). (25)To be submitted for publication. (26)T. Morirnoto and K. Morishige. Bull. Chem. SOC.Jpn., 47,92 (1974). (27)T. Morimoto and K. Morishige, J. Phys. Chem., 79, 1573 (1975).
Resonance Raman Spectra of Monolayers Adsorbed at the Interface between Carbon Tetrachloride and an Aqueous Solution of a Surfactant and a Dye Tohru Takenaka" and Talsuke Nakanaga lnstitute for Chemlcal Research, Kyoto Unlversity, Uji, Kyoto-Fu, 6 11, Japan (Recieved September 5, 1975) Publication costs assisted by Kyoto University
Resonance Raman spectra have been recorded of a monolayer adsorbed at the interface between carbon tetrachloride and an aqueous solution of cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and methyl orange (MO, an anionic azo dye) by using a new method of total reflection of the exciting light at the interface. The adsorbed monolayer consisted of an interaction product of CTAB with MO, and the latter served as an absorber of the exciting light of an Ar+ laser to give rise to the resonance Raman effect. The band frequencies of MO in the monolayer were slightly shifted from those in bulk aqueous solution toward those of solid MO, indicating that the MO molecules in the monolayer were in an environment similar to a crystal field. Theoretical consideration of the Raman scattering activity due to an evanescent wave in the total reflection has been made of uniaxially oriented molecules. From polarization measurements of the resonance Raman spectra, it was found that the long axes of the MO molecules were tilted in the monolayer with a large angle of 50-60° with the axis normal to the interface.
Introduction Oriented monolayers of surface-active agents adsorbed from solution a t the interface between two phases, i.e., liquid-gas, liquid-liquid, or liquid-solid, are the basic concept in surface and colloid chemistry, and spectroscopic study of the adsorbed monolayers in situ has been a subject of much interest in this field. However, previous studies have been very limited in number. Tweet1>' has designed the monolayer spectrometer and obtained visible absorption spectra of monolayers at the air-water interface by means of the multiple reflection of a beam between two parallel mirrors through the interface. It is generally agreed, however, that visible and ultraviolet spectra are too simple in most cases to afford sufficient information on the complex structure of monolayers. The infrared spectrum may be informative, but it is very difficult to apply to studies of monolayers on
liquid substrates, because of an extremely small number of molecules in monolayers and of strong absorption of the substrates. These difficulties are expected to be overcome by the recently developed technique of resonance Raman spectra, in which enormous enhancement of Raman intensity is observed. In addition, there is the advantage of Raman spectroscopy with laser excitation that suitable methods are possible for irradiation of the exciting light upon monolayers at interfaces and for collection of the scattered Raman radiation. In the present work, we succeeded in recording the resonance Raman spectra of monolayers adsorbed at the interface between carbon tetrachloride and an aqueous solution of cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and methyl orange (MO, an anionic azo dye) using a new method of total reflection of the exciting light The Journal of Physical Chemistry, Vol. 80, No. 5, 1976
476
a t the interface. The reasons for the choice of CTAB and MO are as follows. First, a dilute aqueous solution of MO (2 X M) and CTAB (1 X M) has an electronic absorption band around 460 nm,3 and gives rise to the resonance Raman effect for the exciting wavelengths of 488.0 and 514.5 nm of an Ar+ laser.4 The corresponding absorption band of trans-azobenzene has been assigned by Jaff6 et al.536 to the lowest A-A* transition with the transition moment parallel to the long axis of the molecule. Second, the interaction between long-chain alkyltrimethylammonium bromide and MO in dilute aqueous solutions has been studied by Hiskey and D ~ w n e ywho , ~ have pointed out the formation of a 1:l interaction product due to two factors. One is a Coulombic interaction between the negatively charged sulfonic group on MO and the positive charge on the cationic surfactant, and the other is a van der Waals-London type of interaction between the hydrocarbon portions of these two molecules. Finally, it is found in the present work that the interaction product of CTAB with MO is adsorbed a t the interface between carbon tetrachloride and the aqueous solution forming a monolayer, It is noted also that the Raman radiation scattered from a monolayer by this total reflection method is due to an evanescent, nonpropagating wave and not due to a propagating wave as in the usual method. Theoretical consideration of the Raman scattering activity due to the evanescent wave has been made of uniaxially oriented molecules by reference to results about principal values of the derived polarizability obtained from measurements of the depolarization ratios of a bulk aqueous solution. From polarization measurements of resonance Raman spectra, the orientation of the MO molecules in the adsorbed monolayer was discussed.
Experimental Section Materials. A sample of MO was obtained from a commercial source and was recrystallized several times from water. That of CTAB was the guaranteed reagent and was used without further purification. Pure water was prepared by redistillation of distilled water which had been passed through an ion-exchange resin column. Measurements of Interfacial Tension. The interfacial tension between carbon tetrachloride and an aqueous solution was measured by the Wilhelmy plate method using a Shimadzu Model ST-1 surface tensometer and a Teflon plate. Prior to the measurements, the interface was allowed to stand overnight, so that the adsorption equilibrium was established. Measurements of the Resonance Raman Spectra of Adsorbed Monolayers. After some examinations, the total reflection method illustrated in Figure 1 was used. Carbon tetrachloride and a dilute aqueous solution of CTAB and MO were successively poured into a truncated pyramidal Raman cell (placed upsidedown) which was made of glass or acrylic acid resin. A horizontally propagating laser beam was incident upon the lower part of the cell, inside of which carbon tetrachloride was placed. After the refraction a t an inclined optical window of the cell, the laser beam approached the interface between carbon tetrachloride and the aqueous solution through the former with a large incidence angle. Since the refractive index of carbon tetrachloride (1.46) is higher than that of the aqueous solution (1.34), the beam was totally reflected a t the interface and followed by the refraction a t the exit window of the cell. The beam thus emerging from the cell was reflected by a The Journal of Physical Chemistry, Vol. 80, No. 5, 1976
Tohru Takenaka and Taisuke Nakanaga
concave mirror (a laser booster) back through the identical pass with the forward propagation. This was useful to increase the energy of the sample excitation. Both the forward and backward beams were focused upon the interface. The scattered Raman radiation due to the evanescent wave in the total reflection was collected by a condenser lens in a direction perpendicular to the plane of incidence of the exciting light and was led to the monochromator. Another concave mirror (a Raman booster) was provided behind the cell to enable the revival of the Raman radiation scattered backward. This total reflection method has advantages that the optical system is quite simple and that the polarization measurements of Raman spectra are possible as shown later. We now define the space-fixed axes X , Y , and 2 as shown in Figure 1. Apparently, the angle of incidence of the exciting light a t the interface is determined by the angle of inclination of the cell window, and it was designed to form the angle of incidence 84O, while the critical angle is 66.6O. In this case, the penetration depth of the evanescent wave in the aqueous solution was calculated to be 1390 b for the exciting 488.0-nm light of the Ar+ laser.7 The Raman spectra were recorded on a Japan Electron Optics Laboratory Model JRS-S1 laser Raman spectrophotometer with a Spectra-Physics Model 164 Ar+ laser (4 W). The spectral slitwidth was 12 and 14 cm-l for the 514.5and 488.0-nm excitations, respectively. The output power of the exciting light was kept to be less than 100 mW to protect the interface from thermal agitation. The resonance Raman spectra of a bulk solution were obtained using a 1-mm capillary tube.
Results and Discussion Formation o f a Monolayer at the Interface. In Figure 2, the interfacial tension between carbon tetrachloride and aqueous solutions of CTAB and MO is plotted as a function of the logarithm of the concentration of MO in a 1 X M CTAB solution. I t shows a typical interfacial tension-logarithm concentration curve of surfactants, suggesting that the interaction product has surface activity and is adsorbed at the interface. By the use of the Gibbs adsorption isotherm, the surface excess of the interaction product mol/cm2 from the descendis calculated to be 1.9 X ing slope of the linear part of the curve. This means that one interaction product occupies a surface area of 89 b2. The same experiments for interfaces between carbon tetrachloride and aqueous solutions of CTAB alone yielded a value of 43 A2 as the corresponding area for the CTAB molecule, indicating that the contribution of the MO molecule to the surface area occupied by one interaction product is 46 bz.Since this value is reasonable as the area of the MO molecule, it may be concluded that the adsorbed interaction products form the monolayer at the interface between carbon tetrachloride and the aqueous solution. Resonance Raman Spectra of MO i n Bulk Solution and in a Monolayer. Resonance Raman spectra of the bulk M) and CTAB (1X aqueous solution of MO (2 X M) are shown in Figure 3A, where ZII and ZL denote the intensities of Raman radiations with the electric vectors parallel and perpendicular to that of the exciting light, respectively. The Raman bands of MO in aqueous solutions have been studied by Hacker: Carey et al.? and Machida et al.,1° and the frequencies are summarized in Table I together with their assignments. It is found from Figure 3A that all of the observed bands give approximately the same
477
Resonance Raman Spectra of Adsorbed Monolayers Aqueous solution
Slit
Mirror
Aqueous solution
LGz=4
.-In
+
..:. . , ,..., . . .....,...:..:,.,.:.
Z
(A) Aaueous solution
Mirror
beam
Y
h
C c
/
cc14
c
Y
Figure 1. Total reflection method for Raman measurements of monolayers adsorbed at the interface between carbon tetrachloride and aqueous solutions.
20
-
CTAB : I
x lom5M
5
\
0)
1600
15-
1400
1200
-I
IOOC :m
Figure 3. Resonance Raman spectra of MO: (A) aqueous solution of M) and CTAB (1 X M); (B) monolayers adsorbed MO (2 X
.-cc
at the interface between carbon tetrachloride and the aqueous solution. 111 and /A are intensities of Raman radiations with the electric vectors parallel and perpendicular to that of the exciting light, respectively.
UI
10-
I
1 6 ~
I d6 1d5 Concentration of M O
I
,
1 6 ~ M
Flgure 2. Interfacial tension between carbon tetrachloride and aqueous solutions of MO and CTAB as a function of the logarithm of the concentration of MO. The concentration of CTAB is fixed at 1 X 10-5 M.
values around 0.36 of the depolarization ratio ( p l = I l / I i i ) irrespective of the excitation wavelength of the Ar+ laser. Taking account of the experimental error, this value is considered to agree with 0.33, which will be theoretically obtained under the assumptions that the Raman band belongs to the totally symmetric species, and that, of all nonvanishing elements of the derived polarizability tensor, only one diagonal element has an exceptionally large value. l1 Figure 3B represents a resonance Raman spectrum of MO in the monolayer adsorbed at the interface between carbon tetrachloride and the aqueous solution mentioned above. The spectrum was recorded using the total reflection method of Figure 1, and in this case, a polarizer for the Raman radiation was removed so that 111+ 11 can be obtained. Although the scattered Raman radiation is so weak that the S/N value is very low, the Raman spectrum which resembles that in the bulk solution can apparently be observed on the background due to carbon tetrachloride. In
Table I, the Raman frequencies of MO in the monolayer are compared with those in bulk solutions and of solid MO. Evidently, the frequencies in the monolayer are slightly shifted from those in bulk solution toward those of solid MO, suggesting that the MO molecules in the monolayer are in an environment similar to the crystal field. Besides the fact that there is the monolayer at the interface as shown above, additional confirmations of the statement that the Raman spectrum in Figure 3B arises from the monolayer are accomplished by the following examinations. (1)No Raman radiation due to MO can be detected for the interface between carbon tetrachloride and an M), where the adaqueous solution of MO alone (2 X sorption cannot be expected as MO is surface inactive. This may indicate that the evanescent wave penetrated into the aqueous solution does not give rise to detectable Raman radiation whenever the adsorbed monolayer is not formed at the interface.12 (2) The frequencies of the Raman bands in Figure 3B are different from those in the spectra of the bulk solution (Figure 3A), as mentioned above. Differences are also found in the polarization properties of the Raman bands between the bulk solution and the monolayer as will be seen in the later part of this paper. In the next section, we discuss the orientation of the MO molecules in the adsorbed monolayer with the aid of polarization measurements of their resonance Raman spectra. Molecular Orientation of MO i n the Monolayer. We now consider a three-phase plane-bounded system shown in Figure 4. Phase 1 is the semiinfinite incident phase of carbon tetrachloride, phase 2 the adsorbed monolayer, and phase 3 the semiinfinite final phase of the aqueous solution. Since the adsorbed monolayer is much thinner than The Journal of Physical Chemistry, Vol. 80, No. 5, 1976
478
Tohru Takenaka and Taisuke Nakanaga
TABLE I: Raman Spectra of MO in Various States
Aqueous solution
-
Ref 9
Ref 1 0
1597 1449 1423 1415 1393 1373 1318 1294 1201 1151
1600
1450 1420 1395 1320 1200 1155
1118 a
Adsorbed monolayer
Present work
-
1599 1448 1417 1390 1370 1318
1446 1419 1390 1367 1314
1200 1151 1117
11,95 1143 1117
Solida
Assignment b
1592 1443 1419 1412 1391 1368 1313 1292 1197 1145 1118
Benzene ring Benzene ring N=N stretching Benzene ring Benzene ring Ph-N stretching
Reference 9. b Reference 10.
yI Phase 3
, n3
'
I I
Aqueous to~utlon
IIIX
.
I
I .
I
I
= axx21Ex12+ ayx21Ey12
(7)
cc14
Flgure 4. Geometry of total reflection method for Raman measurements of a monolayer at the interface between carbon tetrachloride and an aqueous solution.
=
+ ayu21EyJ2
(8) where the first subscript of I refers to the excitation polarization and the second to that of Raman radiation, and axy etc. are elements of the derived polarizability tensor which is based on the space-fixed axes. Using proper values, n l = 1.46 for carbon tetrachloride, n3 = 1.34 for dilute aqueous solution, and $ = 84O, and making the assumption that n2 = 1.43 for the adsorbed monolayer, from eq 1-3 we have Illy
I.'.
Phase I , n,
(6)
and
:.;,: ,'.,'>:., :.'.I: '.' , ', . ., ., .: ', : : . : ' : ,. ,. . , . .':'Phase ..... 2 , n2 :,'~:~.l;/::.~.'Monolay~r'j~!~ ... ..... ,. , '.:,;.,', j ;:.;: : ,I;.',:.,.:..: ::.;: , , ..::, .I., , ': .::.,*,.':,,'.;.'; I
= azY21Ez12
I l Y
aXY2pX12
lExl2 = 0.04151E1lq2 the penetration depth, the electric field of the evanescent wave in the total reflection can be assumed constant over the monolayer thickness. For the incident polarized lights with electric vectors parallel and perpendicular to the plane of incidence, the electric field amplitudes' in the monolayer are given by
and (3) where n,, = ni/n, is the ratio of the reflective index of phase i to that of phase j , $ the angle of incidence, and Ello and E l o the incoming amplitudes for the parallel and perpendicular polarizations, respectively. EZ represents the electric field amplitude for perpendicular polarization E 1, and that for parallel polarization E I~ is expressed by
El = (IExl2 + IEy(2)1'2
(4)
When the Raman radiation due to the evanescent wave is observed along the 2 axis, four geometries of polarization measurements are available as a consequence of a combination of two polarization directions of the excitation (parallel and perpendicular to the plane of incidence) and two polarization directions of the Raman radiation (parallel to the X and Y axes). The Raman intensities13 obtained by these polarization geometries will be proportional to
The Journal of Physical Chemistry, VoL 80, No. 5, 1976
IEyI2
(9)
0.2161E11q2
(10)
and
IEzJ = 0.2771E 1 q
(11)
The next task is to express the elements axy etc. in terms of elements ary etc. which are based on the principal polarizability axes of the molecule.13J4 To do this, it is convenient to assume a type of orientation of the MO molecule and to use the transformation matrix @ relating the coordinate system X, Y, 2 (designated F ) , and the system x , y, z (designated g) under the assumed type of orientation. Using the elements @pg,we may write aFF'
=
(12)
@Fg@F'g'agg' gg
Here, it is reasonable to assume that the MO molecules in the monolayer are uniaxially oriented with respect to the Y axis which is normal to the interface, the X Z plane. This is indicated in Figure 5, where the long axis of the molecule (the z axis) is rotated around the Y axis with an angle 0, and the molecular x and y axes are free to rotate around the z axis. Under the present assumption of uniaxial orientation, @ . F ~may be given by the direction cosines between the F axes and g axes as functions of Eulerian angles.15 The required terms are obtained by averaging over two Eulerian angles other than 8; one refers to the rotation of the MO molecule around the z axis, and the other to the rotation of the z axis around the Y axis. 2
aFFI2
=
(X@Fg@F/g'agg') gg'
(13)
Since for the MO molecule it has already been known from the measurements of the depolarization ratio of the bulk
479
Resonance Raman Spectra of Adsorbed Monolayers
TABLE 11: Raman Scattering Activities ( a p ~ ’ 2of) Uniaxially Oriented Molecules Expressed as agg2 Coefficients of____-
Y I
Direction of electric field
Monolayer
X
Polarization direction of Raman radiation X
Y
Case 1 (azz>> other elements) light
I
X
Figure 5. Uniaxial orientation of the z axis of MO in monolayers with
Y
respect to the Yaxis.
aqueous solution that only one diagonal element agghas an exceptionally large value as compared with other elements, eq 13 becomes very simple. For two cases in which aizz is largest (case l),and where either axxor ayy is largest (case 2), the results are summarized in Table I1 where expres~ are given as coefficients of sions of six a ~ p in~ eq’ 5-8 agg2.In Figure 6, they are shown as a function of 8. Snyder14 has derived the Raman activities for two special cases (0 = 0 and 90°) of uniaxially orientated molecules without any restriction of the values of aggr’s. Our results coincide with Snyder’s, when ours are applied to these special cases of 8 and a t the same time, Snyder’s are applied to cases 1 and 2. Since the transition moment of the electronic absorption band of MO at 460 nm is assumed to be parallel to the long axis (the z axis) of the molecule,6 all agg”s except azz may vanish, that is, case 1 may be true in this case.ll Thus we have
Zlx = 0.0346 sin4 flElo12azZ2 ZLy =
0.139 sin2 0 cos2 flELo12azz2
(3/8)sin4 e (1/2)sin2 6 cos2 0 (1/8)sin4 6
z
(1/2) sin2 B cos20 cos4 9 (1/2)sin2 8 cos2 0
Case 2 (a,, or aYY >> other elements) (3/64) ( 3 cos4 e + 2 cos2.e + 3) 11/16) sin2 8 13 cosde + I ) (1/64) ( 3 cos4 e + 2 cos2 e + 3)
X Y
(1/16) sin2 0 ( 3 cos2 e + 1) (318) sin4 8
~
z
(1116) sin26 ( 3 cos2 e + 1)
0,8 Case I
I
(14) (15)
Iilx = (0.0156 sin2 8 + 0.108 cos2 0) sin2 ~ E l ~ q 2 a Z(16) z2
I
and
Case 2, Uxx o r a y y ).) Other elements
Illy = (0.0208 sin2 8 + 0.216 cos2 8) cos2flEllq2azz2(17) From eq 14-17 and Figure 6, the following polarization pattern is expected: (1) when 0 = Oo, only a y y 2 has nonzero value and therefore only Illy is observable with relatively large intensity. (2) When 0 = 90°, on the other hand, axx2 and azx2have nonzero values and therefore Zlix and ZLx are observable, although these intensities are as small as y14 and l/S of Illy a t 8 = O o , respectively, if Ell0 = ELo. (3) When 8 is an intermediate value between 0 and 90°, all of the six app2’s have certain values and therefore the Raman spectra can be observed for all of the four polarization geometries. In this case, from eq 14-17, we have the ratios ILX- 1 -- -tan28 ZlY 4
(18)
and
ZIIX - 3 tan2 0 __ Illy 4 tan2 0
+ 20.8 tan2 0 + 41.5
Experimentally, each ratio is obtained by measuring the intensities of the Raman bands recorded by changing the polarization of the Raman radiation in turn from the X to Y directions a t a fixed polarization direction of the exciting light (either perpendicular or parallel to the plane of incidence). Figure 7 represents the polarized Raman spectra of MO in the adsorbed monolayer obtained for the four polarization geometries. For the all geometries, the Raman spectra of MO can apparently be observed on the background due
3”
Figure 6. Raman scatterin activities ( 0 1 ~ ~of7 )uniaxially oriented molecules as a function of case 1, azr >> other elements; case 2 ,
a , or
1:
cyyy
>> other elements (see text).
to carbon tetrachloride.16 This fact suggests that 8 is neither 0 nor 90’ and is an intermediate value. Since, unfortunately, the accuracy of the Raman intensity measurements is not so good owing to the very low S/N value, after more than ten measurements of intensities, the mean values of ~ y obtained to be 0.80 the ratios Zlx/Zly and Z ~ ~ x / Z lwere and 0.77, respectively, for the phenyl-N stretching band at 1143 cm-l. A correction for the polarization character of the monochromator was applied to these values. Substitution of these values into eq 18 and 19 gives the 0 values of 60.8 and 50.3O, respectively. Although it was unable to get one 0 value which simultaneously satisfied both of eq 18 and 19, the magnitude of this discrepancy may be allowed at present, taking account of the low accuracy of the Raman intensity measurements as mentioned above. Thus it may be concluded that the long axes of the MO molecules are tilted in the adsorbed monolayer with a large angle of 50-60’ with the Y axis. An attempt to improve the The Journal of Physical Chemistry, Vol. SO,No. 5, 1976
Francis P. Daly and Chris W. Brown
400
could be obtained about the CTAB surfactant, which may play an important role in the adsorption and formation of the monolayer a t the interface. From this point of view, a succeeding study is in progress by using surface-active dyes which give rise to the resonance Raman effect for the exciting lights of Ar+ laser. The total reflection method described in this paper may be applied to studies of thin layers not only at liquid-liquid interfaces but also a t liquid-gas and liquid-solid interfaces.
References and Notes
iI
IlY
(1)A. G. Tweet, Rev. Sci. lnstrum., 34, 1412 (1963). (2)A. G. Tweet, G. L. Gaines. Jr.. and W. D. Bellamy, J. Chem. Phys., 40, 2596 (1964). (3) C. F. Hiskey and T. A. Downey, J. Phys. Chem., 58,835 (1954). (4) B. Kim, A. Kagayama, Y. Saito, K. Machida, and T. Uno, Bull. Chem.
W a v e n u m b e r , cm-l Resonance Raman spectra of MO in the adsorbed monolayer obtained with four polarization geometries.
Flgure 7.
SOC.Jpn., 48, 1394 (1975). (5) H. H. Jaffe, S. J. Yeh, and R. W. Gardner, J. Mol. Spectrosc., 2, 120 (1956). (6) D. L. Beveridge and H. H. Jaff6, J. Am. Chem. Soc., 88, 1948 (1966). (7)N. J. Harrick, "Internal Reflection Spectroscopy", Interscience, New York, N.Y., 1967,Chapter II. (8) H. Hacker, Spectrochim. Acta, 21, 1969 (1965). (9) P R. Carey, H. Schneider, and H. J. Bernstein, Biochem. Blophys. Res. Commun., 47,588 (1972). (IO)K. Machida, E. Kim, Y. Saito, K. Igarashi, and f. Uno, Bull. Chem. SOC. Jpn., 47,78 (1974). (11) J. Behringer, Observed Resonance Raman Spectra", in "Raman Spectroscopy", H. A. Szymanski, Ed., Plenum Press, New York, N.Y.,
SIN value is now being made by means of the time averaging using a computer in real time. Unipoint multiple reflection techniques" may also compensate for weakness of scattered Raman radiation. It should be noted here that we measured the resonance Raman spectra of the MO dye and could discuss only about the orientation of these dye molecules. No information
1967,Chapter 6. (12)In the monolayer, the number of the MO molecules in a unit volume is as large as a factor of IO4 times that in the bulk aqueous solution.
(13)E. E. Wilson, Jr., J. C. Decius, and P. C. Cross, "Molecular Vibrations", McGraw-Hill, New York, N.Y., 1955,Section 3-6. (14)R. G. Snyder, J. Mol. Spectrosc., 37,353 (1971). (15)Reference 13,Appendix I. (16)The intensity of the background due to carbon tetrachloride depends upon the polarization geometry because of polarization characteristics of the Raman bands of carbon tetrachloride.
(17)N. J. Harlick, Appl. Opt., 5, 1236 (1966).
Raman Spectra of Rhombic Sulfur Dissolved in Secondary Amines Francis P. Daiy and Chris W. Brown* Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 0288 1 (Received June 30, 1975)
Raman spectra of rhombic sulfur dissolved in di-n-butylamine, di-n-propylamine, and dimethylamine have been measured for the first time. Bands due to Sd2- and SS"- were observed in the spectra of the din-butylamine and di-n-propylamine solutions, whereas only bands due to Ssn- appeared in the spectrum of the dimethylamine solution. Comparison of the results on secondary amine solutions with our previous results on primary amines indicates that the ability of secondary amines to form small ionic sulfur species is less than that of primary amines.
The Raman spectra of rhombic sulfur dissolved in ethylenediamine, n-propylamine, and monomethylamine have been reported previously.lJ New bands observed in the 100-600-~m-~region were attributed to the open chain polysulfides SSn-, Sd2-, and S3-. In this note we report the The Journal of Physical Chemistry, Voi. 84 No. 5, 1976
Raman spectra of rhombic sulfur dissolved in the following secondary amines: di-n-butylamine, di-n-propylamine, din-propylamine, and dimethylamine. Raman spectra were measured using a Spex Industries Model 1401 double monochromator with photon counting
